SANAA, Yemen — Hamid al-Ahmar is not a member of Yemen’s ruling party or its military. He holds no formal position in its opposition movement. Nor can he claim the authority of a religious leader.

Yet Ahmar is anything but a mere observer in the seven-month-old populist uprising to oust President Ali Abdullah Saleh. He is a billionaire, a scion of the country’s most powerful tribal family, and he is using his money and power to assert a role in a new Yemen.

He has bankrolled protest marches in 10 provinces, providing everything from microphones to transportation. He commands tens of thousands of tribesmen, including a heavily armed contingent that guards him day and night. His tribe’s clout has bought him access and influence; now it is providing Ahmar with a power base, one that has brought fresh energy to the revolution but has also spawned more violence and chaos.

“I am living with this revolution, day by day, hour by hour,” the 43-year-old said in an interview inside his opulent mansion.

Perhaps more than in any other country in the Middle East, the bonds of the vast extended families known as tribes occupy a central role in Yemen, a country ruled by two rival groupings, the Bakeel and the more powerful Hashid.

But Yemen is hardly alone in the region being riven by tribal loyalties; tribes are a factor in Libya, Jordan, Syria, Saudi Arabia and across the Persian Gulf. In some ways, they play a role just as important as the government, military, clerics and the opposition, injecting another unpredictable dynamic into the turbulence of the Arab Spring.

The Ahmars are blue bloods in Yemen’s tribal society.

Hamid al-Ahmar’s late father, Abdullah, headed the Hashid tribal federation, to which Saleh’s tribe also belonged. Abdullah al-Ahmar also headed the country’s largest opposition party, Islah, and served as speaker of parliament. Hamid’s elder brother now heads the Hashid federation.

In Yemen, tribes make up the central social unit, and their power has only grown in recent years, while Yemen’s central government has proven incapable of controlling much of the country. Most Yemenis depend on their tribes for jobs and other services.

To help maintain his power during more than three decades of rule, Saleh turned again and again to the Ahmars, in a symbiotic relationship not unlike his bond to Maj. Gen. Ali Mohsen, the country’s most powerful military leader — one in which all parties chose to overlook their differences.

In return for help from the Ahmars and Mohsen, Saleh gave them wide latitude to “run their affairs with informal armies, courts and economic empires” and made “direct payments from the treasury to the . . . tribal and military constituencies,” then­U.S. Ambassador Thomas C. Krajeski wrote in a 2005 diplomatic cable released by WikiLeaks.

This year, following the deaths of 52 protesters by snipers loyal to Saleh, the Ahmar clan and Mohsen broke with the president and openly expressed support for the uprising.

‘A fiery combination’

As a child, Hamid al-Ahmar played with Saleh’s sons and nephews. In high school, Ahmar started a tourism company, using family money. He earned an economics degree at Sanaa University and spent two summers in England, where he studied English. He also studied the language in the San Francisco Bay area for four months while visiting one of his nine brothers, Sadiq, who was training to become a pilot.

His business holdings include Yemen’s largest cellphone company, Kentucky Fried Chicken and Baskin-Robbins franchises, a supermarket chain, and an influential satellite-TV network that has been critical of Saleh’s government.

To his supporters, Ahmar represents Yemen’s future: a young, modern businessman with the willpower to guide a nation gripped by poverty and Islamist extremism into a more stable era.

But to his detractors, including many youth activists who sparked the revolution, Ahmar is a living testament to how tribal-backed power has suffocated the Middle East’s poorest nation.

“There is a part of the people who like Hamid al-Ahmar,” said Ali al-Jaradi, editor in chief of al-Ahali newspaper, one of the largest in Yemen. “There are others afraid of what he represents. He’s from the president’s tribe. He’s a part of the old order.”

Unlike his father, though, Hamid al-Ahmar has shown no loyalty to Saleh. As a member of Yemen’s parliament since 1993, he is a senior leader of the opposition party, known as Islah, and he first called on Saleh to step down in 2005 — six years before the uprising that began this year.

“I don’t hate him personally. I hate his way of running the country,” Ahmar said, adding that Saleh’s sons and nephews should also leave power.

His increasing influence did not escape the attention of American diplomats. “Hamid al-Ahmar has ambition, wealth and tribal power in abundance, a fiery combination anywhere but especially in Yemen,” then-U.S. Ambassador Stephen A. Seche wrote in August 2009, in another of the diplomatic cables released by WikiLeaks.

Escalating tensions

On a day in May, armed guards with pickup trucks mounted with machine guns stood guard outside Ahmar’s mansion in the capital’s upscale neighborhood of Hadda.

Throughout the upheaval in the country, the residence has served as a kind of refuge for opposition political leaders who frequently turn to Ahmar for advice, even though he is younger than most of them. The leaders, known as the Joint Meeting Parties, discussed strategy and chewed khat, a leafy narcotic consumed by many men here.

Dressed in a white traditional robe with a large jambiya dagger strapped to his belt, Ahmar bristled when asked whether he and other opposition figures were tainted because of their previously close relationships with Saleh.

“To be fair, without the JMP and without some of these figures, the revolution cannot reach what has been reached,” Ahmar said. “If some people do not appreciate the role of some leaders, they will not appreciate the rights of others.”

The tensions between Ahmar and Saleh escalated in early June after an assassination attempt on Saleh, which forced him to leave the country for Saudi Arabia, where he is recuperating from injuries suffered in the attack.

Soon after that, government forces began shelling Ahmar’s mansion in Hadda, blaming the Ahmars for the attack. The shells struck his house, killing at least 10 of his tribesmen and injuring three dozen.

In the June clashes, Ahmar’s heavily armed tribesmen struck back at the government forces, firing from streets and rooftops. They took over more than a half-dozen government buildings and relinquished control only after the Saudis negotiated a cease-fire.

Fighting also erupted in other areas, including the Hassabah enclave of the capital, where Ahmar grew up in his family’s palatial ancestral compound. The clan denies it played any role in the attempt to kill Saleh.

In the garden of the destroyed family compound, Sadiq al-Ahmar made clear one afternoon in June that the family was divided over his younger brother’s political machinations.

In front of him were charred, bullet-pocked walls. Inside the mansion, shattered glass covered the floors. On the facade of the house, a giant picture of his father remained intact, along with the large emblem of the family’s dagger.

Since the June clashes, the Ahmar tribesmen have not engaged in a more offensive and extensive role in the tribal clashes that have consumed other parts of Yemen. But the environment remains tense. On virtually every street corner of Hassabah, heavily armed Ahmar tribesmen stand guard, fully expecting another assault by government forces.

“We are observers of the state. This is our fate,” Sadiq al-Ahmar said. “Hamid is politically active. I don’t think he’s concerned with becoming president, and I would not prefer him to do this. You are basically carrying a burden. I do not advise him to do so.”

Among others unnerved by the clash between government forces and Hamid al-Ahmar’s tribesmen are youth activists, who fear that their nonviolent call for reform from the streets is being transformed into a power struggle among the nation’s tribes.

“I don’t consider the Ahmars part of the square. What they did in Hassabah separated themselves from our peaceful revolution,” said Riyadh al-Zindani, a student activist. “If the revolution succeeds while the president is outside, it will be called the Hamid al-Ahmar revolution.”

A future Yemen

Publicly, Ahmar plays down his aspirations to lead Yemen one day. He comes from northern Yemen, and he often says the next president should be from the south in order to preserve the unity of Yemen, whose north and south were unified in 1990. He has yet to visit Change Square, where tens of thousands of youth activists have camped for months to demand an end to Saleh’s rule. He understands that he would not be welcome.

“I don’t want anyone to say that Hamid al-Ahmar is coming to take over the revolution,” he explained, “that he wants to make himself the leader of Yemen.”

Yet Ahmar clearly understands the power of the media to effect change. He says he is personally involved in “directing the coverage’’ of his satellite-TV network, and he has worked closely with the Arabic-language al-Jazeera network by “preparing for them a lot of things on the ground” even after the network was banned from the country by Yemen’s government.

He says he understands that his tribal lineage could be perceived as a disadvantage in a new political order and volunteers that in a future Yemen, the tribe should be secondary to the national identity.

“The tribe is not above the law; the interests of the tribe cannot be above the interest of the country,” Ahmar said.

So does he want to become president?

“If they would nominate me, and they think I am the right person,” he said, smiling, “yes, why not?”